Vehicles utilizing hybrid powertrain systems incorporate an engine stop-start function. When the vehicle is stopped in traffic, for example, at traffic lights or slow-moving traffic, the engine operation is discontinued. However, when it is desired to begin moving the vehicle after the stop, the engine must start and the vehicle must begin moving in a substantially seamless manner. There is to be no delay when the operator wishes to accelerate the vehicle along with the remainder of traffic.
The ability to stop and restart the engine improves the overall economy of the vehicle especially in fuel economy. Most hybrid systems will employ at least one torque-transmitting mechanism, such as a clutch or brake, which is engaged when the vehicle is to be launched or begin operation from a stopped position. The time required to fill a servomechanism operating the torque-transmitting mechanism quite generally causes a delay between the engine starting and the vehicle beginning to move.
The hybrid type powertrains, which have been disclosed in the prior art, include a separate hydraulic pump, which is operated to maintain the torque-transmitting mechanism engaged while the engine is not running. When the engine is running, the auxiliary pump operation is discontinued and the more conventional engine-driven system pump operated. While these systems permit the engine operation to be discontinued, the use of a separate pump is energy consuming, space consuming, and more expensive.
The space and mass considerations of the additional pumping mechanism are probably the most serious penalties that are associated with this solution. The space under the vehicle where the powertrain is positioned is very sparse and the addition of any expansion causing assemblies is detrimental or least desirable to the system. Also, the addition of a separate pump mechanism creates an increase in mass for the vehicle and an increased cost.